This invention relates generally to microprocessor communication systems and specifically to a simple serial data transmission system for intercoupling a pair of microprocessors.
Conventionally, when a pair of microprocessors must communicate with each other, one microprocessor is placed in the position of a master and the other in the position of a slave so that control of the communication link or bus connecting the microprocessors may be transferred without confusion. While there are a number of systems for regulating the control of communications buses linking microprocessors, the present invention represents a very simple, economical approach.
The present invention will find use in many situations that require intercommunication between microprocessors. One particular implementation is that of a cable system in which the subscriber station cable converters are microprocessor controlled for receiving downloaded information from the cable head-end. The cable converter may also include an "add-on" on-screen display arrangement or unit for conveying selected information to a subscriber. For example, an out-of-band frequency shift keyed (FSK) type receiver in the add-on unit may receive downloaded channel mapping information in which the channel number displayed at the subscriber station may not be the channel number of the signal actually provided to the television receiver at the subscriber station. This is common in cable systems and is advantageous to the subscriber in that the channel number normally associated with a particular station is retained even though the cable system uses a different number for that station's television signal. In that arrangement, a so-called "conditional access" microprocessor in the subscriber station cable converter is in communication with a so-called "dialogue" microprocessor in the add-on unit and the communications between the two microprocessors must be regulated to enable the conditional access microprocessor to perform its various "off-line" functions while monitoring for downloaded information from the dialogue microprocessor. The dialogue microprocessor may also go "off-line" and perform other tasks for variable time periods, as required. The data protocol used may be conventional. For example, each microprocessor may send successive 24 bit data packets over a data line with data packets being separated by a sync bit, sent over a sync line, to inform the receiving microprocessor that the data packet is finished and to take appropriate action in response thereto.